An X-ray image tube is an electron tube used for medical diagnosis, etc., which converts X-rays into visible lights, etc. The X-ray image tube is constituted of an evacuated envelope as a whole, and an input window is provided at one end of the evacuated envelope, for example at the side from which X-rays enter. The X-ray input window has a periphery secured to a highly strengthened frame which is hermetically sealed to the hollow cylinder portion of the evacuated envelope. Because the inside of the evacuated envelope needs to be kept highly evacuated, the joined portion of the input window to the frame must be highly hermetic.
For a conventional X-ray image tube, it is known that there is a method of attaching an input window part of titanium or titanium alloy which permits X-rays to penetrate, to a frame part of iron alloy, using spot-resistance-welding by means of an intermediate part intervening between them (Refer to Japanese Patent Disclosure Shou/57-3340).
The structure by the above method makes the input window part be swelled out toward the direction of the inside of the evacuated envelope owing to the pressure difference between the inside and the outside of the evacuated envelope, because the thickness of the X-ray input window of titanium or titanium alloy is, for example, 0.1 mm or below. Consequently an input substrate which is swelled out toward the inverse direction, i.e. toward the outside of the envelope, protruding like a dome and has an input screen stuck thereon must be located in the vicinity of the evacuated region inside the concave input window.
In consequence, the entire length of the evacuated envelope becomes elongated. Furthermore, splashes out of the input window, the frame and the intermediate part caused by the resistance-welding are scattered inside the evacuated envelope. The spattering causes some disadvantages such as deterioration of voltage resisting property or occurrence of spot-like traces in an output image.
Another method where an X-ray input part of aluminum or aluminum alloy and a frame part of iron or iron alloy plated with nickel (Ni) layer on the surface thereof are welded to each other by thermo-compression welding is also known(Refer to Japanese Patent Publication Shou/58-18740).
Referring to FIG. 10 to FIG. 12, an X-ray image tube whose input window part and frame part are welded to each other by thermo-compression welding, and its thermo-compression welding method will be explained.
In FIG. 10, evacuated envelope 101 is constituted of X-ray input window 102 located at one end of the envelope to permit X-rays to pass through, hollow cylinder portion 103 positioned at the center, output window 104 located at the other end of the envelope, etc. Input window 102 is welded to frame 105 of high mechanical strength metal at the periphery thereof and frame 105 is joined to hollow cylinder portion 103. On the inner surface of input window 102 on the evacuated side, input screen 106 converting X-rays into electrons is directly stuck. Inside evacuated envelope 101, a plurality of focusing electrodes 107a to 107c which accelerate and concentrate electrons emitted out of input screen part 106, and anode 108 are provided. On the evacuated side of output window 104, output screen 109 which converts electrons into predetermined output signals is formed. Mark M denotes the tube axis.
Referring to FIG. 11, welding method of the joined portion of input window 102 to high mechanical strength frame 105 surrounded by the circle A of FIG. 10 will be explained. In FIG. 11, each part corresponding to that in FIG. 10 is denoted by the same mark as that in FIG. 10, and repeated explanations will be partially omitted.
Mark 111 denotes the cylindrical holder of the welding device. Ring shaped frame 105 is put on holder 111. Frame 105 is made of stainless steel, whose cross section is bent like a crank as shown in the figure, and plated with nickel on the surface. Frame 105 is comprised of first flat portion 105a at the inside, perpendicular portion 105b bent perpendicularly from first flat portion 105a and second flat portion 105c at the outside. Next, flange portion 102f of the periphery of input window 102 is located so as to contact the upper surface of first flat portion 105a of frame 105. Input window 102 is made of aluminum(Al) alloy for example, and has domed shape protruding toward the upside of the drawing. Then press punch 112 contacts flange portion 102f of the periphery of input window 102 from upside thereof.
Under the construction mentioned above, the pressure of about 1600 kg/cm2 is supplied to the contact region of input window 102 with frame 105 to bond them, while holder 111 and press punch 112 are heated to about 500 degrees centigrade.
The aforementioned thermo-compression welding method is performed under the condition of high temperature and heavy pressure. Therefore, the frame part and the input window part are prone to deform. Especially, because aluminum material which is the material for the input window part flows in large quantity toward both the inner area and the outer area of the pressed region, the input window probably deforms seriously in the vicinity of the joined portion.
Namely in FIG. 12, when the shape of input window 102 before being welded to frame 105 (the shape of the input window under the condition of FIG. 11) is denoted by the dot line D, there is a tendency that input window 102 after thermo-compression welding deforms as the inner region of the joined portion swells like the shape denoted by mark E. This region includes the effective region in which the input screen is formed. Owing to the occurrence of deformation in such region, distortion in an electronic lens formed in the evacuated envelope takes place partially, when the input screen is formed directly on the inner surface of input screen 102.
Moreover, deformation such as wrench often takes place by heating high mechanical strength metal frame 105 to which input window 102 is welded. There is moreover some probability that the above phenomenon makes the deformation of the input window after being welded to each other become remarkable. Namely input window 102 with high precision can hardly be formed by the thermo-compression welding method, so that some contrivance should be required to ameliorate a production yield.
In conventional X-ray image tubes, as explained heretofore, when titanium alloy is used as the input window material, the input window becomes depressed toward the inside of the evacuated envelope, because the thickness of titanium alloy plate is very thin. Therefore the entire outline of the tube becomes bigger, because electrodes must be properly located therein. This causes some difficulty in miniaturizing an X-ray diagnostic apparatus accommodating an X-ray image tube. When aluminum alloy is employed for input window material, the region to be welded is heated to high temperature while the input window and the frame are being welded to each other. Owing to the deformation of the frame and the input window, distortion in the electronic lens formed in the evacuated envelope takes place. Consequently the resolution of the output image deteriorates partially. Therefore further improvement has been expected.
An object of the present invention is to provide an X-ray image tube which can suppress or prevent any deformation of the X-ray input window by adopting the structure where the input window and the frame are hermetically welded to each other by ultrasonic welding to overcome the shortcomings in the conventional technology mentioned above, and a manufacturing method thereof.